1. Field of the Invention
The invention relates to nanowire-based optoelectronic devices for the generation of light, especially LEDs (light-emitting diodes).
2. Description of Related Art
The “planar” technology is the technique currently implemented to form light-emitting devices, such as LEDs, based on III-V, and in particular III-N, material, and on II-VI material, such as GaN, ZnO, or GaAlAs, for example, emitting in the blue spectral domain or for a conversion into white light.
A LED in planar technology is usually formed by successive epitaxies, especially by MOCVD (“Metal-Organic Chemical Vapor Deposition”), of layers of semiconductor materials from the III-N family.
Thus, referring to FIG. 1, which is a simplified view of a planar LED 10 of the state of the art, a GaN layer 12, n-doped with silicon, is deposited on a sapphire substrate 14. An active layer 16, formed of multi-quantum wells made of an alternation of unintentionally doped GaN and InGaN sub-layers 18 and 20, is deposited on n-doped GaN layer 12. A p-doped AlGaN electron blocking layer 22, commonly called “EBL”, is further deposited between active layer 16 and a GaN layer 24 p-doped with magnesium. Finally, lower and upper electric contacts 26 and 28 are respectively formed on layer 12 and on p-doped layer 24 for the electric connection of LED 10.
Thus, electrons injected into active layer 16 by means of n layer 12 and holes injected into active layer 16 by means of p layer 24 at least partly radiatively recombine in active layer 16, the quantum multi-wells having a confinement function, as known per se. Light is thus emitted by active layer 16.
The planar LED based on III-N semiconductors and on quantum wells, such as previously described, suffers from performance limitations.
Problems of electric injection and of electric confinement of the carriers are first posed.
Indeed, on the one hand, the hole mobility is very small as compared with the electron mobility, and on the other hand, holes are injected at a lower concentration than electrons due to the difficulty of activating magnesium atoms, here, the acceptors, in p-doped GaN layer 24 and to the higher resistivity of layer 24 with respect to layer 12. EBL layer 22 is thus necessary to locate radiative recombinations of electron-hole pairs in the InGaN/GaN quantum wells.
EBL layer 22 however requires to be designed with the utmost care, especially as concerns its energy bands, its hetero-epitaxial growth on a layer 16, and its ternary material composition. Indeed, a poorly designed AlGaN layer results in blocking the holes injected by p-doped layer 24 and thus in making LED 10 inefficient.
Problems of internal quantum efficiency of planar LED 10 are then posed.
A significant decrease of the LED efficiency can be observed for current densities greater than 10 A/cm2 due to a phenomenon known as “Droop Efficiency”, which thus rules out LEDs from many applications requiring a high current density greater than 200 A/cm2, such as for example displays or lightings.
More specifically the droop efficiency phenomenon is associated with the sources of loss by non-radiative recombination of electron-hole pairs, among which the following can be mentioned:                the displacement of charge carriers outside of indium-rich regions in InGaN/GaN multi-quantum wells;        losses due to local dislocations and defects;        losses by thermal effect;        the significant piezoelectric polarization between InGaN/GaN heterostructures, which generates a spatial separation of electron-hole pairs and the appearing of interface states;        Auger recombinations, by direct or phonon-assisted mechanisms; and        an inefficient carrier injection due to electrons escaping from the multi-quantum wells and to the low hole concentration in these wells.        
The losses are all the greater as the carrier density is high.
To decrease the droop efficiency, and thus increase the current density applicable to a LED with no significant decrease of its efficiency, the density of charge carriers in the radiative recombination area of electron-hole pairs should be decreased.
For this purpose, N. F. Gardner et al.'s article, “Blue-emitting InGaN-GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200 A/cm2”, Applied Physics Letters 91, 243506 (2007), provides replacing the multi-quantum wells, which, by nature, induce a droop efficiency even for low current density values, with a double heterostructure such as shown in FIG. 2.
As illustrated in this simplified cross-section view of a planar LED 30, the InGaN/GaN multi-quantum wells are here replaced with a single unintentionally doped InGaN layer 32, which forms with layers 12 and 22 a double heterostructure 34.
As compared with the LED with multi-quantum wells 10 of FIG. 1, double heterostructure LED 30 has an increased efficiency for high current densities on the order of 200 A/cm2. Indeed, the increase of the volume of InGaN material, where electron-hole pair recombinations occur, results in decreasing the charge carrier density, which is the main cause of the droop efficiency. This improvement has thus been demonstrated on devices emitting around 440 nanometers with a double GaN/InGaN heterostructure having a 10-nm thickness and containing approximately 14% of indium.
However, the planar LED with a double heterostructure 30 also suffers from fundamental limitations.
First, the generic problems of electric injection and of limited internal quantum efficiency specific to planar LEDs are encountered, that is, EBL layer 22 is necessary, and thus poses the same problems as previously described, and the volume of active area 32 is decreased as compared with the total volume of LED 30. Also, although double heterostructure 34 effectively solves problems inherent to the structure in the form of multi-quantum wells, it has its own specific problems.
Indeed, due to the large mesh parameter difference between the InGaN of layer 32 and the GaN of layer 12, that is, a difference of approximately 10%, it is difficult to epitaxially grow InGaN material with a high indium concentration and/or a large thickness. Indeed, beyond a thickness called “critical thickness”, crystal defects appear within the InGaN material, which defects cause a substantial loss of internal quantum efficiency due to the non-radiative recombinations that they generate.
Thus, to obtain a high current density in LED 30 by increasing the InGaN volume, layer 32 should have a low indium composition, which limits the wavelengths capable of being emitted to the blue spectrum.
The planar double heterostructure thus introduces a strong antagonism between the LED emission wavelength and the possible current density with no efficiency loss.
Concurrently to planar LED technology, LEDs based on InGaN/GaN nanowires, especially manufactured by epitaxial growth, especially by MBE (“Molecular Beam Epitaxy”) epitaxy, or by MOCVD epitaxy, are known.
Two categories of LEDs based on nanowires can be distinguished in the state of the art:                those where the active area of the nanowires comprises confinement structures having multi-quantum wells with an axial epitaxial growth, that is, along the nanowire growth axis,        and those where the active area of the nanowires comprises confinement structures having multi-quantum wells with a radial epitaxial growth, that is, in a volume formed around the nanowire growth axis.        
FIG. 3 schematically shows in cross-section view an example of nanowire 40 with axial-epitaxy multi-quantum wells. Nanowire 40 is formed of a GaN area 44 n-doped with silicon, formed on an n+-doped silicon substrate 42, having an active area 46 formed of axial multi-quantum wells formed of an alternation of unintentionally doped GaN areas 48 and InGaN areas 50 formed thereon. A GaN area 52, p-doped with magnesium, is further deposited on an EBL area 54, itself deposited on active area 46.
According to this axial geometry, the electrons and the holes are injected into active area 46 respectively by means of substrate 42 and of area 52, and recombine, at least partly radiatively, in active area 46.
FIG. 4 schematically shows in cross-section view an example of nanowire 60 with multi-quantum wells grown by radial epitaxy on an n+-doped silicon substrate 62. Nanowire 60 comprises a GaN core 64 n-doped with silicon, surrounded with an active area 66 formed of radial multi-quantum wells formed of an alternation of unintentionally doped GaN areas 68 and InGaN areas 70. An EBL volume 74 surrounds active area 66, EBL volume 74 being itself surrounded with a GaN volume 72 p-doped with magnesium. Areas 66, 74, and 72 are further formed on an electric insulation layer 76.
According to this radial geometry, the electrons and the holes are injected into active area 66 respectively by means of substrate 62 and of area 72, and recombine, at least partly radiatively, in active area 66.
The nanowires, and more specifically their manufacturing method, have a number of advantages, among which:                a growth of nanowires on substrates, each formed of a material with a mesh parameter mismatched with the other. Thus, silicon, which is a conductive low-cost substrate, capable of being manufactured in large sizes, may be envisaged for the growth of nanowires made of III-N material, which is impossible in planar technology. This variation has advantages both in terms of production cost and of simplification of manufacturing methods, especially in terms of electric injection;        a good crystal quality due to the stress relaxation at the free surfaces. Thus, a decrease of the number of non-radiative recombination centers, and especially the absence of through dislocations, can be observed as compared with planar structures; and        a better extraction of light with no complexification of manufacturing methods.        
On the other hand, LEDs based on nanowires are less limited in terms of wavelength to be emitted than planar LEDs, since the alloy composition range forming the active layer can be expanded.
However, the LEDs based on nanowires just described also suffer from fundamental limitations.
First, whatever the geometry adopted for a nanowire-based LED of the state of the art, an EBL area is necessary to confine the carriers. Thus, in the same way as for planar LEDs, a growth perfectly controlled both in terms of morphology, of composition, of thickness, and of doping of the binary and ternary III-N semiconductors of the EBL area is indispensable.
Further, the active area here again has a decreased volume with respect to the total volume of the nanowire, which implies a limited internal quantum efficiency.
Finally, the active areas of nanowire-based LEDs of the state of the art appear in the form of multi-quantum wells. Thus, even though a better droop efficiency behavior of such LEDs as compared with planar diodes with multi-quantum wells of the state of the art would be observed, the presence of multi-quantum wells nonetheless implies a limited current density applicable to LEDs before their efficiency substantially decreases.
Document WO2009/106636 also discloses a LED based on nanowires. The nanowires, made of n-type ZnO, are epitaxially grown on a ZnO buffer layer deposited on a silicon substrate. The n-doped ZnO nanowires are further embedded in a p-doped semiconductor polymer layer, especially PEDOT/PSS, and two metal electrodes are respectively in contact with the ZnO buffer layer for electron injection and with the polymer layer for hole injection. A large p-n surface junction is thus obtained between the n-type ZnO of the nanowires and the p-type polymer layer due to the nanowire geometry.
However, the volume of active material of this type of LED, that is, the volume of material where electrons and holes radiatively recombine, is very small since this volume is limited to the interface of the p-n junctions formed of the n-type nanowires and of the p-type polymer layer.
Thus, to date, there exists no LED enabling to have, at the same time, a high current density, a high internal quantum efficiency, and a large freedom of choice as to the emitted wavelength.